Patent Application
20240418882
The invention relates to systems and methods for designing seismic surveys to enable seismic data acquisition with a reduced environmental impact and lower survey costs. The methods include incorporating a controlled random decimation of seismic receivers or sources along a fixed or varying distance on seismic lines, or over a specified area.
Seismic surveys are a method of exploration geophysics used to estimate the properties of the Earth's subsurface from reflected seismic waves. Generally, seismic surveys are conducted across survey areas wherein seismic data is acquired by imparting a seismic source of energy into the ground and then listening for reflected energy at a number of nearby receivers.
Seismic waves are mechanical perturbations that travel in the Earth at a speed governed by the acoustic properties of the medium in which they are travelling. The acoustic (or seismic) impedance of a medium, Z, is defined by the equation:
where v is the seismic wave velocity and ρ (rho) is the density of the rock.
When a seismic wave travelling through the Earth encounters an interface between two materials with different acoustic impedances, some of the wave energy will reflect off the interface and some will refract through the interface. Thus, and fundamentally, the seismic reflection technique consists of generating seismic waves and measuring the time taken for the waves to travel from the source, reflect off an interface and be detected by the array of receivers at the surface.
Knowing travel times from the source to various receivers, and the velocity of the seismic waves, software is used to reconstruct the pathways of the waves to build up an image of the subsurface.
Seismic surveys are conducted over wide areas of land and water as a means of understanding underlying geological formations.
Currently, almost all oil and gas exploratory wells are preceded by 3D seismic surveys. The basic method of a 3D survey is the same as for a 2D survey; however, in a 3D survey, instead of a single line of energy source stations and receiver stations, the source stations and receiver stations are laid out in a grid across the ground surface. As a result, recorded reflections that are received at each receiver point come from all directions, requiring sophisticated computer programs to analyze this data to create a three-dimensional image of the subsurface.
As noted, 3D surveys are acquired by laying out energy source stations and receiver stations in a grid over the land area to be surveyed. The receiver stations are set to record the reflected vibrations from the source stations and are typically laid out in parallel lines (receiver lines). The source stations are also laid out in parallel lines that are generally perpendicular to the receiver lines, but may be in other orientations, with regular or irregular spacing. The spacing of the source and receiver stations and lines is determined by the design and objectives of the survey.
Energy sources for a seismic survey are most commonly either subsurface explosive charges or surface vibroseis, although a seismic survey is not limited to using these specific energy sources, only that the seismic energy input into the subsurface needs to be sufficient to travel downwards and reflect off the target of interest and return to surface with sufficient energy to be distinguished from background noise.
3D surveys are conducted over a large area in order to provide sufficient data for accurate interpretation of the subsurface geology. Surveys are conducted over all types of land including rural land and remote land. Depending on the location, any type of surface feature may be present on the surface including any type of forest, vegetation and/or man-made features. Conducting a survey requires the physical placement of sources and receivers at known locations. Given the need to prepare ground to ensure safe placement of sources and proper placement of receivers, each source and receiver requires access to specific ground locations by both personnel and heavy equipment. Moving teams of personnel and equipment deep into remote areas is challenging, requiring cutlines to be cut through forest to enable this access.
3D surveys conducted at different times and covering different but adjacent areas can later be combined into a single data set for processing and analysis, provided there is sufficient overlap of the areas covered by the two surveys.
In the past, when a survey was being planned, for simplicity, source and receiver lines were simply designed as cut lines orthogonal to one another, which has an impact on the environment. That is, machinery would be deployed to the field and would cut broad and uniform lines through forest/vegetation and/or plow/push lines through various habitats to enable the source and receiver lines to be set up.
A full seismic survey representation is conducted with regular sampling of the source and receiver stations and lines. Randomization enables the quantity of source stations and receiver stations used to be reduced but can result in an irregular distribution of source and receiver stations and/or lines throughout the survey area. By randomizing the receiver stations and source stations used in a controlled setting, a similar set of data can be produced when compared to a full representation with fewer sources and explosive devices required, allowing for lowered costs. Randomization has favorable properties for seismic data reconstruction algorithms which leads for better imaging of subsurface targets. Current technology conducts randomization of an entire geographical area, resulting in potential areas being overrepresented or underrepresented in the data, instead of randomizing groups of sources and receivers to equalize the spread throughout the geographical area being tested.
A method is provided of designing seismic surveys for a geographical area using controlled randomization. The method comprises the steps of selecting the geographical area to be surveyed; determining placement for one or more control areas, wherein each control area comprises a plurality of stations, within the geographical area; applying a randomization algorithm to each of the one or more control areas; creating a Fresnel fold map of the geographical area; and applying a relative standard deviation algorithm to the Fresnel fold map to confirm if the randomization algorithm has created an even distribution of data.
A method of designing seismic surveys for a geographical area using controlled randomization. The method comprises the steps of selecting the geographical area to be surveyed; determining placement for one or more control areas, wherein each control area comprises a plurality of stations, within the geographical area; and applying a randomization algorithm to each of the one or more control areas.
Various objects, features and advantages of the disclosure will be apparent from the following description of particular embodiments of the disclosure, as illustrated in the accompanying drawings. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of various embodiments of the disclosure. Similar reference numerals indicate similar components.
Various aspects of the disclosure will now be described with reference to the figures. For the purposes of illustration, components depicted in the figures are not necessarily drawn to scale. Instead, emphasis is placed on highlighting the various contributions of the components to the functionality of various aspects of the disclosure. A number of possible alternative features are introduced during the course of this description. It is to be understood that, according to the knowledge and judgment of persons skilled in the art, such alternative features may be substituted in various combinations to arrive at different embodiments of the present disclosure.
With reference to the figures, a method for seismic survey planning and implementation is described. The method described enables improved survey planning and design of seismic surveys to reduce the number of seismic receiver stations (sensors) and/or source stations, while maintaining a uniform coverage of study area without big gaps.
The control aspect involves applying the randomization over a control area, rather than applying randomization over the entire geographical area. There can be multiple control areas within the overall geographical area to be surveyed, and the control area or areas can take up all or some of the entire geographical area to be surveyed. This control area can be defined by any one of the following:
It is possible to have any combination of the above control area types within a single overall geographical area to be surveyed. The randomization is repeated across a part or all of the geographical area to be surveyed (in-line, crossline, areal, or a combination). A station interval is the interval between consecutive source and receiver stations. A Fresnel Zone determines the horizontal resolution of seismic surveys at a specific depth. Areal geometry comprises a control area of a smaller area of the geographical area but is larger than one salvo.
The result is a geographic area with a similar footprint to the geographic area to be surveyed, but with fewer sources and receivers used. The reduction in sources and receivers can be defined by a number of sources and receivers to remove per control area or by the percentage of sources and receivers to be removed per control area. The percentage of sources and receivers to remove may be rounded up or down to a whole number. Also, the controlled randomization may be applied over a limited region of the survey area instead of the entire survey area.
The method described by the disclosure designs a seismic survey for a selected geographical area with a controlled randomized distribution of sources and receivers. This method may be applied to a geographical area to determine the installation location of sources and receivers and to determine the distribution of data collection points from the receivers and sources. The method may also be applied to a geographical area where sources and receivers have already been installed to determine which receivers and sources will be used for data collection. Furthermore, this method can be applied in overlapping and/or adjacent geographical areas across the entire survey or limited to specific geographical areas. The controlled randomization allows for decimation of data by reducing the number of data collection points on the receiver lines and source lines, respectively. The decimation of the data and distribution of sources and receivers may be variable over the geographical area.
Sources generate seismic waves after an event occurs, such as setting off controlled explosives. The seismic waves emitted from the source are recorded by nearby receivers. The sources and receivers are typically set up into source lines and receiver lines to create an orthogonal grid. Referring to
According to an embodiment of the disclosure, applying a controlled randomization algorithm to distribute data for each receiver line by decimating the data between salvos, ensures local regions of the geographical area retain a consistent minimum distribution of data collection and avoids creating local regions of heavy data collection, which would lead to a misrepresentation of the data collected. In the present method, the source or receiver stations are decimated along each source or receiver line (or within a specified area) using a controlled randomization algorithm which decimates the data between each salvo, or using Fresnel zone size (or another distance or area metric), making sure that the decimation does not lead to big gaps in the survey coverage. The resulting survey using the salvo-controlled randomization on receivers is shown in
It is possible to apply the randomization algorithm to a control area only once, or singly, to determine a decimation of source or receiver stations. It is also possible apply the randomization algorithm to a control area multiple times, or in sequence, before selecting a most suitable and satisfactory decimation and distribution of stations.
Applying a randomization algorithm to the receivers without any control creates areas that are overrepresented, with some areas between salvos being fully represented and other areas having only one, or even no, receiver installed. The use of a pure random decimation strategy is referred to in
Referring to
To analyze the suitability of randomized seismic surveys for seismic data reconstruction algorithms, offset-limited Fresnel fold maps of the surveys are generated.
A relative standard deviation (RSDF) parameter is introduced after the randomization algorithm has been applied to the sources and receivers to quantitatively measure the differences between the Fresnel fold maps, which is calculated by
Where μF is the mean of all data within a survey area and σF is the standard deviation of the data from the Fresnel fold map. A smaller RSDF value indicates a more uniform distribution of the Fresnel zone coverage. In the case of applying the randomization algorithm to the control area in sequence, it is also possible to create a Fresnel fold map and to determine RSDF value for each time the randomization algorithm is applied, to determine a most suitable and satisfactory distribution of stations. In this way the method is repeated as needed until an acceptable relative RSDF value is achieved.
The method can be applied to any type of seismic geometry including, but not limited to: 2D, 3D, 4D, orthogonal, slant, megabin, linear, sinusoidal, zig-zag, areal, swath. As well, the method can be performed with any type of randomization algorithm as it is not dependent on the type of randomization, but rather the distribution of the randomization.
When designing a survey, it is important that the source and receiver density is adequate to image the subsurface target of interest. The base survey provides a base starting point to selectively move or delete sources and receivers having consideration to the environmental and other surface or subsurface data.
Although the present disclosure has been described and illustrated with respect to preferred embodiments and preferred uses thereof, it is not to be so limited since modifications and changes can be made therein which are within the full, intended scope of the disclosure as understood by those skilled in the art.
